compiler doesn't flag multiple calls to consuming func

[eecs441staff]

Description

When struct S below is declared ~Copyable, Xcode 15.3 beta tags its instance (s) as being consumed more than once. But if S is declared without ~Copyable, Xcode doesn't give an error message and the consuming function value() can be called more than once on the instance.

Reproduction

struct S: ~Copyable {
    let v = "hi"
    consuming func value() -> String { return v }
}

func f() {
    let s = S()   // Xcode 15.3 beta error message: 's' consumed more than once
    print(s.value())
    print(s.value())
}

f()

but:

struct Sc  { // or class
    let v = "hi"
    consuming func value() -> String { return v }
}

func g() {
    let s = Sc()             // no error message
    print(s.value())
    print(s.value())      // prints out a second time
}

g()

Expected behavior

Xcode gives the same error message in both cases and the second call to s.value() an error (as per SE-0377).

Environment

Xcode 15.3 beta on macOS 14.4 beta CLI.

Additional information

No response

[tbkka]

This is expected. The compiler is allowed to freely make copies of a Copyable type. I've added comments below to clarify how this works in this case:

func g() {
    let s = Sc()
    // Because `s` will be used again, the compiler will actually
    // create a copy for this first call.  That copy gets consumed here:
    print(s.value())
    // The original value can then be consumed here:
    print(s.value())
}

[eecs441staff]

Thanks. I realized a copy has been made after posting the issue. The following prevents the compiler from making a copy of s:

func h(_ s: consuming Sc) {  // error: s consumed more than once
    print(s.value())   // consumed here
    print(s.value())   // consumed again here
}
h(Sc())

Perhaps there Is a compiler flag/Xcode setting that gives a warning when a copy is made for a value about to be consumed? Thanks.